1. Field of the Invention
The present invention relates to a communication system comprising a communication line monitor, and more particularly, to a communication system comprising plural terminal apparatuses and a communication line monitor for monitoring a communication line. The present invention also relates to a communication line monitor for monitoring a communication line connected to plural terminal apparatuses, and further, to a method for monitoring a communication line connected to plural terminal apparatuses.
2. Description of the Prior Art
FIG. 8 shows a communication system comprising two terminal apparatuses 81a and 81b which are connected to a communication line 13.
Referring to FIG. 8, the terminal apparatus 81a comprises a terminal unit 84a, a terminal controller 83a for controlling the terminal unit 84a, and a terminal communication controller 82a connected to the terminal controller 83a and to the communication line 13. Similarly the terminal apparatus 81b comprises a terminal unit 84b, a terminal controller 83b for controlling the terminal unit 84b, and a terminal communication controller 82b connected to the terminal controller 83b and to the communication line 13. Further, in order to display packet data of one frame transmitted between respective terminal communication controllers 82a and 82b, there is provided a line monitor 85 comprising a communication controller 86 for receiving packet data transmitted from a terminal communication controller 81a or 81b through the communication line 13, a display controller 88 for controlling a display unit 89, the display unit 89 for displaying contents of the packet data, and a table section 87 having a RAM operably connected to the display controller 88 for storing a table of a correspondence relationship between packet data and character code corresponding to the packet data (referred to as a correspondence table hereinafter).
FIG. 5 shows one example of packet data transmitted between respective terminal apparatuses 81a and 81b.
Referring to FIG. 5, one frame of packet data is composed of a master address MA, a slave address SA, a control field CF, an operation code OPC and an operand OPR. The two byte master address MA represents an address of a terminal apparatus 81a or 81 b which transmits packet data, and the two byte slave address SA represents an address of another terminal apparatus 81b or 81a which receives the packet data. The one byte control field CF represents an attribute of the packet data. In the case of transmitting a command, the control field CF is set at "0A"h (h denotes a hexadecimal number), which denotes an operation "WRITE COMMAND AND LOCK" for locking the occupation of a receiving terminal apparatus and writing a command therein. The operation code OPC and the operand OPR represent command data to be transmitted to the receiving terminal apparatus, where the one byte operation code OPC denotes a type of command and the operand OPR including detailed contents of the operation code OPC.
One example, in which the terminal apparatus 81a is a video tape recorder No. 0 having an address "0120"h and the terminal apparatus 81b is a video system controller No. 0 having an address "0128"h, will be described below.
When the terminal apparatus 81a of the video tape, recorder No. 0 transmits a "POWER ON" command to the terminal apparatus 81b, i.e. the video system controller No. 0, the master address MA of the packet data transmitted is "0120"h, the slave address SA thereof is "0128"h, the control field CF thereof is "0A"h, the operation code OPC thereof is "A0"h, and the operand OPR is "70"h. In this case, the terminal unit 84a instructs the terminal communication controller 82a through the terminal controller 83a to transmit this command to the terminal communication controller 82b of the terminal apparatus 81b, and then, the terminal communication controller 82a transmits the above-mentioned packet data to the terminal communication controller 82b of the terminal apparatus 81b. Then the terminal communication controller 82b instructs the terminal controller 83b to turn on (i.e. ("Power ON") the terminal unit 84b.
During this communication operation, the communication controller 86 of the line monitor 85 receives the transmitted and received packet data in a manner similar to that of the terminal communication controller 82b, and then, outputs such data to the display controller 88. Thereafter, the display controller 88 displays the received packet data in a hexadecimal number form on the display unit 89 as shown in FIG. 6. Further, in order to display character codes of the received packet data, the display controller 88 analyzes the packet data by referring to the table section 87 showing a correspondence between the packet, data and the character codes, converts the packet data into corresponding character codes, and displays the converted character codes on the display unit 89 as shown in FIG. 7.
In FIGS. 6 and 7, frame No. 01 corresponds to the packet data of the above example, and the contents shown in FIG. 6 correspond the contents shown in FIG. 7, respectively. Namely, the master address "0120"h corresponds to a character code "VTR0", the slave address "0128"h corresponds to a character code "VTS0", the control field "0A"h corresponds to a character code "WC+lk", the command "A0"h corresponds to a character code "POWER", and the command "70"h corresponds to a character code "N".
Thus, the display controller 88 of the line monitor 85 displays the character codes on the display unit 89 by analyzing the received packet data by referring to the correspondence table stored in the table section 87 to thereby convert the packet data into the character codes.
However, in the above-mentioned conventional communication system, in the case where a terminal apparatus having a function of processing a new command is added to the communication system, such that a new type of packet data is added, or such that the packet data is altered, the line monitor 85 which refers to the original correspondence table stored in the table section 87 can not process such new or altered types of packet data, namely, can not convert such data into character codes for and display the display unit 89.